This invention relates to ovens for preheating tubular parisons or the like in preparation for a blow molding operation and in particular to a means for improving the uniform application of heat to the parisons.
One method of making blow molded articles involves the extrusion of a plastic material in the form of a tube. The tube is severed into discrete lengths called parisons and cooled. The individual parisons are then reheated to a predetermined temperature, one end is pinched off and the parison is blown to the final article shape in a mold.
Depending upon the article being made and the material of which it is to be made, the reheat process can be quite critical and difficult. For example, a method of making blown articles from polypropylene is described in considerable detail in commonly owned U.S. Pat. 3,765,813. That patent describes an endless conveyor which passes through an oven. Rotatable support means for carrying parisons in a vertically oriented position are mounted on the conveyor. Toothed wheels are fixed to the support means and engage upstanding pins which are arranged adjacent the path of movement of the conveyor. The engagement of the pins with the toothed wheels causes the parison support means to rotate as they pass through the oven. Examples of such parison rotating mechanisms are described in more detail in U.S. Pat. Nos. 3,149,373 and 3,740,868.
U.S. Pat. No. 3,947,243 recognized the problem that if there is any misalignment between the conveyor and/or toothed wheel and the vertical pins for engaging the wheel, there is the danger of the wheel not rotating or the wheel or pin breaking. In order to avoid such damaging interference between the spoked wheel and the vertical pin or rotating mechanism, the aforesaid U.S. Pat. No. 3,947,243 suggests the use of stiff vertically or horizontally arranged bristles alongside the conveyor to engage the teeth of the wheel.
While this system assures continued rotation of the parison as it passes through the oven and avoids interference or breakage of parts, bending of individual wires forming the bristles causes rotors to rise up out of the bristles and the continued rubbing of a multitude of bristles on the toothed wheel causes wear on the teeth. It is, therefore, desirable to provide a means for rotating the parisons which avoids these problems.
In accordance with the present invention, an oven of the type having an endless conveyor passing therethrough and including rotatable parisons support mechanisms having a toothed wheel is provided with an improved means positioned alongside at least portions of the conveyor for engaging the toothed wheels of the parison holders for turning these wheels about their axis as the parison holders move through the oven. In accordance with the main feature of this invention, the tooth engaging means combines the characteristics of stiffness sufficient to accomplish rotation of the parison holder and the capability of yielding to forces exerted thereon by misalignment between the teeth of the toothed wheel and the tooth engaging means thereby eliminating abrasive engagement between the tooth engaging means and the tooth.
This is accomplished by providing the tooth engaging means in the form of a multiplicity of coil springs vertically arranged along the length of the conveyor in tooth engaging position. The coil springs can bend and distort a certain amount to accommodate any misalignment of the tooth or conveyor yet are ridged enough to pop into the spaces between the teeth and cause rotation of the parison holder. Further, the fact that the relatively smooth individual coil elements which engage the sprocket extend parallel to the line of motion of the sprocket results in elimination of the abrasive engagement between the individual coil elements and the toothed wheel.
It is an object, therefore, of the present invention to provide in an oven for heating tubular parisons an improved means for assuring that the parisons are uniformly heated about their circumferences and through their wall thicknesses.
It is another object of the present invention to provide an improved means of rotating the parisons as they traverse through an oven which will have an extended service life.
It is still another object of the present invention to provide resilient means for engaging toothed wheels for rotating parisons mounted on a conveyor passing through an oven which exhibits reduced wear or abrasion on the toothed wheel.
Other objects will be in part apparent and in part pointed out more in detail hereinafter.
The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application of which will be indicated in the appended claims.